Torque motor driven spool valves are well known in the art including those which operate through the utilization of a rotary torque motor having a drive member extending eccentrically from the rotor thereof into contact with the spool valve. The drive member directly reciprocates the spool within a bore provided in the valve housing to thereby control the flow of fluid from a source thereof to the load in response to electrical signals applied to the drive motor. Typical of such direct drive servovalves are those shown in U.S. Pat. Nos. 4,793,377; 4,197,474; 2,769,943; 2,697,106; 4,452,423; 4,339,737; 4,702,123, Canadian Patent 601808 issued July 1960 and United Kingdom Patent 1521668 issued Aug. 16, 1978.
As is readily noted from the typical prior art, direct drive servovalves incorporate a housing within which there is disposed a sleeve within which in turn is disposed a spool which moves reciprocally. The sleeve is typically manufactured of hardened steel and the spool of aluminum. Titanium or steel is typically used for the housing. Large annuli are formed in the sleeve to maximize the flow capacity for a given spool diameter and to provide for symmetric sleeve porting to pressure balance the spool radially. The associated intricate sleeve machining, critical sizing of the sleeve and fitting it to the housing and the assembly process required to substantially eliminate leakage between the sleeve and the housing add substantially to the manufacturing cost of such valves.